Integrated circuits, including computer chips, are manufactured by building up layers of circuits on the front side of silicon wafers. An extremely high degree of wafer flatness and layer flatness is required during the manufacturing process. Chemical-mechanical planarization (CMP) is a process used during device manufacturing to flatten wafers and the layers built-up on wafers to the necessary degree of flatness.
Chemical-mechanical planarization is a process involving polishing of a wafer with a polishing pad combined with the chemical and physical action of a slurry delivered onto the pad. The wafer is held by a wafer carrier, with the backside of the wafer facing the wafer carrier and the front side of the wafer facing a polishing pad. The polishing pad is held on a platen, which is usually disposed beneath the wafer carrier. Both the wafer carrier and the platen are rotated so that the polishing pad polishes the front side of the wafer. A slurry of selected chemicals and abrasives is delivered onto the pad to affect the desired type and amount of polishing. (CMP is therefore achieved by a combination of chemical softener, physical downward force, and rotation that removes material from the wafer or wafer layer.) The downward force, referred to in this application as the Spindle Force, is split in the wafer carrier to a Retaining Ring Force and a Wafer Force.
Using the CMP process, a thin layer of material is removed from the front side of the wafer or wafer layer. The layer may be a layer of oxide grown or deposited on the wafer, a layer of metal deposited on the wafer, or the wafer itself. The removal of the thin layer of material is accomplished so as to reduce surface variations on the wafer. Thus, the wafer and layers built-up on the wafer are very flat and/or uniform after the process is complete. Typically, more layers are added and the chemical mechanical planarization process repeated to build complete integrated circuit chips on the wafer surface.
A variety of wafer carrier configurations are used during CMP. One of these configurations, such as Strasbaugh's Variable-input Pneumatic Retaining Ring (ViPRR) Carrier, is designed to hold the wafer to the carrier inside the boundary of the retaining ring while an inflatable seal situated behind the retaining ring is pressurized. The inflatable ring seal extends the retaining ring into the polish pad generating the Retaining Ring Force. An equation or look-up table is used to determine the amount of air pressure required in the inflatable ring seal to generate a certain amount of force on the ring while the remaining spindle force is exerted against the wafer.
Another configuration of wafer carrier manufactured by Strasbaugh and used in CMP is designed to have the retaining ring fixed to the carrier while an inflatable membrane is used to apply pressure behind the wafer. The inflatable membrane behind the wafer generates the force acting on the wafer called the wafer force. An equation or table is used to determine the amount of air pressure required in the membrane to apply a specified force to the wafer during polishing.
Spindle force on CMP tools is created by the use of a pivoting mechanism coupled to a spindle and actuated by a bellows, piston or other actuator means. Currently, spindle force on CMP tools is calibrated periodically to ensure the spindle force applied during CMP is accurate. A technician uses a load cell fixture to measure spindle forces at various bellows or piston pressures and inputs this information into the controlling computer for calibration. The downward spindle force generated by pressures in a bellows actuation system can change over time, so periodic calibration is required to determine the corresponding spindle force generated by the bellows. The CMP tool must be taken out of service to perform this calibration.
Today, there is no convenient way for measuring spindle force, wafer force, or retaining ring force. Presently, the equation calibrating inflatable seal pressure to ring force or membrane to wafer force, depending on the carrier type, is pre-determined experimentally at the factory using a load cell fixture. Forces are measured for a reasonable sampling of inflatable seals or membranes, depending on the wafer carrier type, at various air pressures. From these experiments, a generic factory equation is calculated and this equation is used for all wafer carriers of that type. As a result, there are many generic equations covering various types and sizes of wafer carriers.
Many problems are encountered using this method of calibration due, in part, to manufacturing inconsistencies between inflatable seals and membranes. Since membranes and inflatable seals are made of a rubber-like material (Such as EPDM, Silicone, HNBR, Buna, etc.) using traditional molding methods, dimensional tolerances are relatively large. In addition to dimensional variations, there can be many differences in the material properties due to composition inconsistencies from seal to seal and membrane to membrane. Also, material properties and dimensions can change over time due to various conditions. Some of these conditions include cycling stresses caused by continuous inflation and deflation, chemical attack by the slurry, heat cycles, and exposure to air and moisture. The dimensional and material properties of the inflatable seals and membranes greatly affect the force calibration curve and changes to these properties can have adverse effects to the calibration curve. Due to the manufacturing inconsistencies, the material inconsistencies, and changes in properties over time, the generic factory force calibrations for wafer carriers are not completely accurate. This can result in sub-optimal and inconsistent polish results.
Previously, semi-conductor designers and manufacturers lived with the inconsistencies in surface flatness, designing their chips around the issues. Other designers and manufacturers required tighter tolerances. These organizations would handle the problems through individual characterization and sorting of the membranes and inflatable seals using a custom bench test machine. This process is slow and labor intensive. Many inflatable seals are deemed un-usable and scrapped because they do not fall within certain predetermined limits. With wafer tolerances becoming more critical methods and devices that are able to quickly characterize and calibrate individual inflatable seals or membranes prior to or in between polishing runs are needed to ensure accurate wafer and retaining ring forces are used in wafer processing.